This application claims the benefit of German patent application DE 199 14 285.8, filed Mar. 30, 1999, herein incorporated by reference
The invention relates to a method and apparatus for sizing a yarn sheet, wherein in a first step wetting agent, which is at a temperature of 65 to 95xc2x0 C., is applied to the yarn sheet, which is then squeezed until it contains a selected residual moisture, and in a second step a sizing agent, which is at a temperature of 65 to 95xc2x0 C., is applied to the yarn sheet, which is then squeezed until it contains a selected residual moisture.
In preparation for weaving, a sizing is applied to warp yarn in the form of a yarn sheet. In the process, the yarn sheet is passed through a sizing bath, is squeezed thereafter and finally dried.
It is known from DE 42 37 962 to pass a warp yarn sheet through a water bath prior to the sizing bath, and to subsequently squeeze the yarn sheet following the water bath. The water bath is heated above room temperature and contains chemical additives. This pre-treatment results in the finishing being pre-loosened and partially washed out. It also improves the absorption capability of the warp yarns.
It is also known from DE 42 34 279 to apply sizing to a sheet of fine-capillary yarn of the micro-filament type, wherein the yarn is pre-wetted in a first step with a liquor which is diluted in comparison to the final concentration of the sizing liquor and is squeezed in a second step until it contains a first residual moisture, and immediately following the squeezing is treated with a sizing liquor at an end concentration, and subsequently is squeezed until it contains a second residual moisture.
The wetting agent is essentially water, containing little or no sizing. The wetting agent is heated to a temperature of close to approximately 90xc2x0 C. A yarn of the micro- filament type can be produced by this method, which has a core relatively low in sizing or containing no sizing, as well as a sheath sufficiently stable for weaving and containing a relatively large amount of sizing agent.
For successful sizing it is necessary to maintain a defined sizing concentration and a defined sizing temperature in the sizing bath. With too high a sizing concentration, too much sizing is used up, which is undesirably expensive. On the other hand, too low a sizing concentration can lead to problems during subsequent weaving. A defined sizing temperature of, for example, 85xc2x0 C., is required for setting a viscosity of the sizing bath which assures an even sizing application.
In connection with single-stage sizing without pre-wetting, Applicant knows to set the sizing concentration and the sizing temperature by supplying steam under ambient pressure and adding sizing liquor, hereinafter called sizing agent, to the sizing bath. This setting is performed in such a way that the sizing liquor removed with the yarn sheet and the water, which evaporates because of the high sizing temperature, are continuously replaced.
Applicant furthermore knows to provide the sizing unit with a double sheath heated by steam under ambient pressure. Because of this the amount of steam introduced directly into the sizing bath for heating can be reduced.
If the yarn sheet is pre-wetted in a first step, and sized in a second step, the yarn sheet is already provided with the wetting agent and slightly cooled when it reaches the sizing bath. The pre-wetted yarn sheet absorbs less sizing agent than a dry yarn sheet. When sizing after pre-wetting, more energy is needed for heating the yarn sheet provided with the wetting agent on the one hand, but on the other hand less sizing agent is needed. In the course of heating the sizing bath by supplying steam, a considerable dilution of the sizing concentration can occur at start-up, if no compensating steps are taken. If the sizing concentration is set in such a way that it attains its desired value after start-up, the yarn sheet is charged with too great an amount of sizing at start-up. In this case the initial amount lies above the desired amount of sizing by 15 to 20%, and the desired amount of sizing is only reached after the first 2000 to 3000 m of yarn sheet.
It is the object of the invention to further develop a method and apparatus for sizing a yarn sheet in such a way that the normal significant dilution of sizing concentration during start-up is minimized.
This object is attained by indirectly heating the sizing agent, preferably by a contact surface at a temperature of 100 to 160xc2x0.
It is important that energy is supplied indirectly for heating the sizing agent instead of steam which condenses to water in the sizing bath, thereby diluting the size concentration. Several possibilities of indirect heating of the sizing agent in the size box can be employed, such as one or several heating panels or heating coils arranged in the size box, which are heated by a heating medium, such as steam under pressure, or hot oil, or electricity. It is also important that a temperature of at least 100xc2x0 C. be maintained at the contact surface by the heating medium, which is at a temperature of greater than 100xc2x0 C. Temperatures between 120 and 140xc2x0 C. are particularly advantageous.
Indirect heating of the sizing agent allows the setting of the sizing temperature without the supply of water. Only a defined amount of wetting agent, mainly water, is introduced into the sizing bath by the replacement of wetting agent with sizing agent in the yarn sheet. A defined amount of water evaporates from the sizing bath because of the high sizing temperature. In comparison with the amount of the evaporated water, only a small excess of the amount of water introduced by means of the yarn sheet leads to the dilution of the sizing bath during start-up. The lowering of the sizing concentration during start-up is less than 10%, for example, wherein the equilibrium of the sizing concentration will already be reached after a few hundred meters of the yarn sheet. It is possible to accept such slightly increased initial amounts of sizing agent, namely less than 10%, so that the sizing concentration can be set by adding sizing agent alone. Additional steps, such as the measurement of the sizing concentration and regulation of the sizing application on the basis of weight measurements described, for example, in DE 42 34 279, or by matching the sizing concentration by means of a concentration mixer, are no longer required.
A temperature of the wetting agent in the first step, for example 5 to 10xc2x0 C. higher than the temperature of the sizing agent in the second step, i.e. the sizing temperature, reduces the temperature difference between the yarn sheet already provided with wetting agent and slightly cooled, and the sizing temperature. The yarn sheet provided with the wetting agent can be heated more rapidly, resulting in less energy being required for heating.
Preferably, cooling of the yarn sheet wetted in the first step until it is acted upon by the sizing agent in the second step is maximally limited to 10xc2x0 C. Limiting the cooling can take place by the second step immediately following the first step. This can be made possible by a compact construction of the sizing device. The limitation of cooling can also be provided by insulating the yarn sheet by means of a cover and/or the supply of heat to the yarn sheet on the way from the wetting agent bath to the sizing bath. The limitation of cooling assures the low energy consumption in the second step.
In the preferred embodiment, in the first step after pre-wetting the yarn sheet, it is squeezed so it contains a residual moisture between 25 and 60%. The residual moisture corresponds to the weight of the water as a function of the weight of the yarn sheet. The residual moisture of the yarn sheet following sizing and squeezing in the second step is set, as described in DE C 42 34 279, to be the same, but in most cases higher. A comparatively low residual moisture of the yarn sheet after the first step prevents a strong dilution of the sizing bath, adds to a stable sizing concentration and has an advantageous effect on the energy consumption because of the small amount of wetting agent which needs to be heated.
In the apparatus of the present invention, which is suitable for carrying out the method of the present invention, the sizing unit has an indirect heater with a flat contact surface.
Depending on the amount of heat output required, the contact surface constitutes a portion of or the entire bottom of the sizing box, wherein the contact surface of the indirect heater is integrated into the bottom of the size box. This integration and the flatness of the contact surface assure the even heating of the sizing agent in the size box without the danger of sizing agent baking onto a transition edge or onto irregularities. A simple and compact structure of the size box, and therefore of the entire sizing device, is made possible by the integration of the contact surface into the bottom.
Preferably, the indirect heater has a heating plate designed as a steam pressure panel consisting, for example, of two plates which are connected with each other in places and, has a low structural height. Its surface can moreover easily be matched to the required heat output.
It is possible to obtain a compact structure of the sizing apparatus by use of a common housing for the wetting unit and the sizing unit, which is divided into a wetting agent box and a size box by a separating wall.
A compact structure is also obtained by the size box being arranged directly downstream of the wetting agent box and with its bottom at the approximate height of the last roller of the wetting unit. In this case the first roller of the sizing unit is located closely downstream of and slightly above the last roller of the wetting unit. The length of the reach of the yarn sheet between the wetting unit and the sizing unit is reduced by this compact construction. The minimal length of the reach prevents excessive cooling of the yarn sheet. Therefore, only a comparatively small supply of energy from the indirect heater is required for heating the sizing agent. This permits a comparatively small heating panel, for example, which can be advantageously accommodated in the sizing agent bath.
A cover over the yarn sheet reach and preferably a heater in the cover, are further features which present excessive cooling of the yarn sheet on its way from the wetting unit to the sizing unit.
The preferred embodiment of the invention is illustrated in the accompanying drawings and diversified in detail thereafter.